The magnetic behavior of eight samples of biotite and a lepidomelane was studied in the temperature range 1.5-300 K and analyzed in relation to their chemical composition. The platy morphology of biotite facilitates magnetic measurements with the applied field either in the plane of the sheets (X ~ ) or perpendicular to the plane (X ~ ). At high temperature, the samples obey Curie-Weiss laws, showing typical paramagnetic behavior which is due to the presence of both Fe 2 + and Fe3+ ions. However, the susceptibilities are anisotropic lrllx~ '" 1.25), because of a trigonal contribution to the crystal field which affects only the ferrous ions. Formulae are given which permit the ferrous Curie-Weiss parameters to be derived from the measured susceptibility.At low temperature (T < 10 K), three of the biotite samples containing high concentrations of iron (17 to 27 wt. %) undergo an antiferromagnetic transition. The magnetic structure consists of octahedral sheets of iron ions with their moments ferromagnetically coupled in the plane, while the interplane interactions are antiferromagnetic and roughly 30 times weaker. Thus, biotite has both magnetic and structural two-dimensional aspects. We also have observed magnetic aftereffect, and some parasitic ferromagnetism below the transition.PACS numbers: 75.50.Kj J.
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